Gas hydrate formation inhibitor and method for inhibiting gas hydrate formation with the same

ABSTRACT

A gas hydrate formation inhibitor consisting of an amphiphilic polymer (such as N-isopropylmethacrylamide (co)polymer) which bears nonionic groups (such as hydroxyl groups) at the polymerization-initiation and -termination ends and has a weight-average molecular weight of 500 to 10,000; and a method for inhibiting gas hydrate formation by adding the gas hydrate formation inhibitor to a system wherein a gas hydrate is to be formed. According to this invention, the formation of gas hydrates can be inhibited, and gas hydrates can be stabilized from the viewpoints of theories of chemical equilibrium and rate process.

TECHNICAL FIELD

[0001] The present invention relates to gas hydrate (methane hydrate,etc.) formation and dissociation-controlling agents and to a gas hydrateformation and dissociation-controlling method.

BACKGROUND ART

[0002] It is known that keeping aqueous media which contain dissolvedgas molecules including carbon dioxide gas or hydrocarbons such asmethane and ethane at a specific temperature and pressure produces “gashydrates”, defined as icy crystals wherein the dissolved gas moleculesare surrounded by water molecules. Gas hydrates formed during mining andshipping of crude oil and natural gas often constitute a cause pipelineclogging, creating a major obstacle to safe and continuous operation.

[0003] Gas hydrates are also known to be naturally present underhigh-pressure, low-temperature conditions. For example, studies haveconfirmed the existence of huge reserves of methane gas hydrates(hereinafter referred to as “methane hydrates”) in vast zones underpermafrost in cold regions such as Siberia or Alaska, or a few hundredmeters or more below the sea floor. In recent years, methane hydrateshave become the focus of attention as an energy source with lowemissions of carbon dioxide and nitrogen or sulfur oxides, which are asource of environmental pollution, and a safe method for retrievingnatural methane hydrates in a stable state has therefore been desired.

[0004] LNG (Liquid Natural Gas) methods are usually employed fortransport and storage of fuel gas and especially methane gas, butbecause of the high construction and building costs for LNG bases andLNG transport tankers, projects on large gas fields with substantialreserves are generally planned on the assumption of extendedreimbursement periods. LNG is therefore not suitable for small-scale gasfields and can even be an impediment to development of small-scale gasfields. The use of gas hydrates for transport and storage of natural gasis considered to be more cost effective than LNG in the case ofsmall-scale gas fields, and the cost can be further reduced bystabilizing the storage of gas hydrates under milder conditions by usingadditives and the like.

[0005] Thus, it is desirable to inhibit or delay formation of gashydrates during pipeline transport of water-containing drilled gas suchas methane, while it is also desirable to accelerate and stabilizeformation of extracted gas hydrates during their shipping and storageand to accelerate dissociation and/or inhibit formation of gas hydratesduring extraction of the gas hydrates from the sea floor or the ground.In order to stabilize or delay dissociation of gases such as methanewhen their gas hydrates are utilized as storage means, gas hydrateformation and dissociation controlling agents must satisfactorilyexhibit the following apparently contradictory aspects of performance.

[0006] (1) They must inhibit formation of gas hydrates (equilibriumformation inhibition) or delay their formation rate (kinetic formationinhibition, or formation delay).

[0007] (2) They must accelerate formation of gas hydrates (equilibriumstabilization, or kinetic formation acceleration) or delay thedissociation rate of formed gas hydrates (kinetic stabilization, ordissociation delay).

[0008] International Patent Publication WO98/53007 describes variousadditives and stabilizers for formed gas hydrates which function toinhibit formation and growth of gas hydrates and/or aggregation ofunstable nucleus structures at the initial stage of gas hydrateformation, and this publication teaches that a polymer composedprimarily of an N-alkyl (meth)acrylamide-based monomer and anN,N-dialkyl (meth)acrylamide-based monomer, having a weight-averagemolecular weight in the range of 400-7000 and a cloud point of 50° C. orhigher with a distilled water concentration of 1 wt %, is effective forcontrolling formation of gas hydrates. Specifically, such polymersinclude acryloylpyrrolidine homopolymers, acryloylpiperidinehomopolymers andisopropylacrylamide/2-acrylamido-2-methylpropanesulfonate copolymers.However, as polymers in the specified molecular weight range areessentially oligomers, the properties of the ends of the polymers arenot controlled, despite the fact that they are known to have a majoreffect on the properties of the polymer molecule as a whole.

[0009] International Patent Publication WO97/07320 teaches thatamphipathic polymers with a certain structure exhibit a high effect ashydrate inhibitors (hydrate formation inhibitors). However, thispublication nowhere mentions the properties of the polymer end groups orthe method of producing the polymers.

[0010] International Patent Publication WO96/41786 describes a processusing N-isopropylmethacrylamide and N-methyl-N-vinylacetamide with thenon-ionic initiator azobisisobutyronitrile, using an amphipathic polymercopolymerized in benzene as the gas hydrate inhibitor. However, as theamphipathic polymer used has a high weight-average molecular weight,almost no effect is produced by both of its non-ionic ends.

[0011] As examples of additives with gas hydrate formation-acceleratingeffects, particularly using methane gas, there may be mentioned thealiphatic amines, alcoholic cyclic compounds and tetrahydrofuransdescribed in Japanese Unexamined Patent Publication No. 4-316795,Japanese Unexamined Patent Publication No. 6-17089, Japanese UnexaminedPatent Publication No. 6-25021 and Japanese Unexamined PatentPublication No. 9-49600. Such additives, however, are all low molecularweight compounds and, therefore, when dissociating hydrates to obtaingas, it is difficult to completely separate the additives due to reasonsof vapor pressure. In addition, the hydrate stabilizing effect isunsatisfactory.

[0012] Japanese Unexamined Patent Publication No. 10-216505 describes anadditive for hydrate formation using a surfactant containing a siliconeresin, but this publication relates to a method for facilitating contactbetween gas molecules and water by lowering the water surface tension ina hydrate-forming system using a silicone resin-containing surfactant,and it provides no effect of accelerating formation of, or stabilizing,the gas hydrate structure.

[0013] Also, an example of Japanese Unexamined Patent Publication No.10-338715 mentions polyacryloylpyrrolidine with a molecular weight of5,000, produced using the non-ionic2,2′-azobis(cyclohexane-1-carbonitrile) (V-40, product of Wako PureChemical Industries) as a polymerization initiator. A gas hydrateinhibitor is also mentioned in this publication as an example of use ofthe polymer. However, this example merely describes production of thepolymer, while the performance of the resulting polymer as a gas hydrateinhibitor is not confirmed. In addition, numerous ionic and non-ionicpolymerization initiators are cited for use as prior art, of which theV-40 used in the example is merely one, and therefore the use of ionicpolymerization initiators is allowed. Consequently, Japanese UnexaminedPatent Publication No. 10-338715 cannot be considered to disclose a gashydrate formation and dissociation-controlling agent comprising anamphipathic polymer, wherein the polymerization initiation andpolymerization termination ends of the polymer are non-ionic and theweight-average molecular weight is in the range of 500-10,000.

DISCLOSURE OF THE INVENTION

[0014] It is an object of the present invention to provide gas hydrateformation and dissociation-controlling agents and a gas hydrateformation and dissociation-controlling method exhibiting both thefunction of inhibiting formation of gas hydrates and the function ofequilibrium and kinetic stabilization of gas hydrates.

[0015] The present inventors achieved the invention upon finding thatamphipathic polymers having a specified weight-average molecular weightand specific properties for the polymerization ends of the polymerexhibit very excellent performance as gas hydrate formation anddissociation-controlling agents.

[0016] The invention therefore provides gas hydrate formation anddissociation-controlling agents which are amphipathic polymers havingnon-ionic polymerization initiation and polymerization termination endsof the polymers, and having weight-average molecular weights in therange of 500-10,000. Such amphipathic polymers are preferably obtainedby (co)polymerization of amphipathic monomers.

[0017] The invention further provides a gas hydrate formation anddissociation-controlling method which comprises adding any of theaforementioned gas hydrate formation and dissociation-controlling agentsto a gas hydrate-forming system.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a schematic block diagram of an apparatus for evaluationof gas hydrate formation and dissociation-controlling performance.

BEST MODE FOR CARRYING OUT THE INVENTION

[0019] An amphipathic polymer used for the invention has non-ionicpolymerization initiation and polymerization termination ends and has aweight-average molecular weight in the range of 500-10,000. Here,“amphipathic polymer” is a general term for any polymer having ahydrophobic group and a hydrophilic group, and it may be obtained by,for example, homopolymerization of an amphipathic monomer,copolymerization of an amphipathic monomer and a hydrophilic orhydrophobic monomer which is copolymerizable therewith, orcopolymerization of a hydrophilic monomer and hydrophobic monomer. Themethod for producing the amphipathic polymer is preferably a method of(co)polymerizing an amphipathic monomer, that is, a method of obtaininga homopolymer by homopolymerization of an amphipathic monomer or amethod of obtaining a copolymer of a copolymerizable hydrophilic orhydrophobic monomer with an amphipathic monomer. In the case of acopolymer, the proportion of the amphipathic monomer component in thepolymer is preferably 10-99 mole percent, and more preferably 50-90 molepercent.

[0020] For the purpose of the invention, an amphipathic monomer is onehaving both a hydrophilic group and a hydrophobic group, and also havinga polymerizable group. For example, it is “a monomer soluble in waterand soluble in solvents that are not miscible with water (commonlyreferred to as non-aqueous solvents)”, which is also polymerizable. Forthe invention, however, monomers which are not definitely amphipathicalone but are amphipathic as polymers will also be referred to as“amphipathic monomers” for convenience.

[0021] As examples of amphipathic monomers there may be mentionedN-ethyl (meth)acrylamide, N-cyclopropyl (meth)acrylamide, N-isopropyl(meth)acrylamide, N-n-propyl (meth)acrylamide, N-methyl-N-ethyl(meth)acrylamide, N,N-diethyl (meth)acrylamide, N-methyl-N-isopropyl(meth)acrylamide, N-methyl-N-n-propyl (meth)acrylamide,N-(meth)acryloylpyrrolidine, N-(meth)acryloylpiperidine, N-2-ethoxyethyl(meth)acrylamide, N-3-methoxypropyl (meth)acrylamide, N-3-ethoxypropyl(meth)acrylamide, N-3-isopropoxypropyl (meth)acrylamide,N-3-(2-methoxyethoxy)propyl (meth)acrylamide,N-3-(2-methoxyethoxy)propyl (meth)acrylamide, N-tetrahydrofurfuryl(meth)acrylamide, N-1-methyl-2-methoxyethyl (meth)acrylamide,N-1-methoxymethylpropyl (meth)acrylamide,N-(2,2-dimethoxyethyl)-N-(meth)acrylamide,N-(1,3-dioxolan-2-ylmethyl)-N-(meth)acrylamide,N-2-methoxyethyl-N-(meth)acrylamide, N-2-methoxyethyl-N-n-propyl(meth)acrylamide, N-2-methoxyethyl-N-isopropyl (meth)acrylamide,N,N-di(2-(ethoxyethyl) (meth)acrylamide, N-vinylpyrrolidone,N-vinylcaprolactam, N-isopropenylpyrrolidone, N-isopropenylcaprolactamand the like.

[0022] Among these, N-ethyl (meth)acrylamide, N-cyclopropyl(meth)acrylamide, N-isopropyl (meth)acrylamide, N-n-propyl(meth)acrylamide, N-methyl-N-ethyl (meth)acrylamide, N,N-diethyl(meth)acrylamide, N-methyl-N-isopropyl (meth)acrylamide,N-methyl-N-n-propyl (meth)acrylamide, N-(meth)acryloylpyrrolidine,N-(meth)acryloylpiperidine, N-vinylpyrrolidone, N-vinylcaprolactam arepreferred, with N-isopropyl methacrylamide being particularly preferred.

[0023] The hydrophilic monomer has the properties of high interactionand high affinity with water, while also having a polymerizable group,and it will typically be a polymerizable water-soluble monomer.

[0024] As examples of hydrophilic monomers there may be mentionedN-(meth)acrylamide, N-methyl (meth)acrylamide, N,N-dimethyl(meth)acrylamide, N-(meth)acryloylmethyl homopiperazine,N-(meth)acryloylmethylpiperazine, N-2-hydroxyethyl-N-(meth)acrylamide,N-3-hydroxypropyl (meth)acrylamide, N-2-methoxyethyl (meth)acrylamide,N-3-morpholinopropyl (meth)acrylamide, N-(meth)acryloylmorpholine,N-2-methoxyethyl-N-methyl (meth)acrylamide, (meth)acrylic acid and itssalts, 2-hydroxyethyl (meth)acrylate, ethyleneglycol (meth)acrylate,diethyleneglycol (meth)acrylate, polyethyleneglycol (meth)acrylate,propyleneglycol (meth)acrylate, butanediol (meth)acrylate,trimethylolpropane (meth)acrylate, dimethylaminoethyl (meth)acrylate,dimethylamidopropyl (meth)acrylamide, vinyl acetate, vinyl propionate,methyl vinyl ether, ethyl vinyl ether, 2-vinyl-2-oxazoline,2-vinyl-4-methyl-2-oxazoline, 2-isopropyl-2-oxazoline, N-vinylacetamide,N-vinyl-N-methylacetamide, N-vinylformamide, N-vinyl-N-methylformamide,N-vinyl-N-n-propylpropionamide, N-vinyl-N-methylpropionamide,N-vinyl-N-i-propylpropionamide, N-vinylpropionamide, vinyl butyrate,N-allylamide, maleic acid, vinylimidazole, dimethylaminoethyl(meth)acrylate methylchloride, dimethylaminoethyl (meth)acrylatebenzylchloride, 2-(meth)acrylamido-2-methylpropanesulfonic acid and itssalts, (meth)acrylamide methanesulfonic acid and its salts,(meth)acrylamide ethanesulfonic acid and its salts,2-(meth)acrylamido-n-butanesulfonic acid and its salts, glycosyloxyethylacrylate, glycosyloxyethyl methacrylate, glycosyloxyethyl-α-ethylacrylate, glycosyloxyethyl-β-methyl acrylate,glycosyloxyethyl-β,β-dimethyl acrylate, glycosyloxyethyl-β-ethylacrylate, glycosyloxyethyl-β,β-diethyl acrylate, ethylene glycol,propylene glycol and the like.

[0025] Preferred among these monomers are N-(meth)acrylamide, N-methyl(meth)acrylamide, N,N-dimethyl (meth)acrylamide,N-(meth)acryloylmorpholine, N-2-methoxyethyl-N-methyl (meth)acrylamide,(meth)acrylic acid and its salts, 2-hydroxyethyl (meth)acrylate,ethyleneglycol (meth)acrylate, polyethyleneglycol (meth)acrylate,propyleneglycol (meth)acrylate, dimethylaminoethyl (meth)acrylate, vinylacetate and 2-(meth)acrylamido-2-methylpropanesulfonic acid.

[0026] As examples of hydrophobic monomers there may be mentioned alkyl(meth)acrylate, alkyl (meth)acrylamide, heterocyclic (meth)acrylates,heterocyclic (meth)acrylamides, and vinylbenzenes optionally havinglower alkyl groups or halogen atoms as substituents on the benzenerings. In particular, polymers partially consisting of monomer unitsderived from alkyl (meth)acrylate, alkyl (meth)acrylamide, heterocyclic(meth)acrylates or heterocyclic (meth)acrylamides are preferred as gashydrate formation and dissociation-controlling agents from thestandpoint of interaction with gas molecules in water by intermolecularforces.

[0027] There are no particular restrictions on the method for producingthe amphipathic polymer, and for example, there may be mentioned methodssuch as aqueous solution polymerization, solution polymerization usingorganic solvents, bulk polymerization, precipitation polymerization,emulsion polymerization, reverse-phase emulsion polymerization,soap-free polymerization, suspension polymerization, reverse-phasesuspension polymerization, and the like, using amphipathic monomersalone or amphipathic monomers with other monomers such as hydrophilicmonomers as the starting materials. Among these polymerization methods,aqueous solution polymerization, solution polymerization using organicsolvents, bulk polymerization, precipitation polymerization, emulsionpolymerization and soap-free polymerization are particularly preferred.

[0028] For control of gas hydrate formation, it is important to controlthe interaction between the hydrophobic gas molecules and watermolecules. It may be possible to more suitably control the interactionbetween gas molecules and water molecules by means of a random copolymerthan a block copolymer of a hydrophilic polymer and amphipathic polymer.Consequently, a copolymer of a hydrophilic monomer and an amphipathicmonomer is preferred from the standpoint of controlling the balancebetween hydrophilicity and hydrophobicity.

[0029] The solvent used for the polymerization may be appropriatelyselected depending on the polymerization method, and in most caseswater, alcohols, acetic acid esters and ethers may be used. Sincesolvent-derived end groups are introduced into the polymer by chaintransfer during polymerization, the solvent is preferably one which doesnot result in introduction of ionic end groups into the polymer.However, even when an ionic solvent is used for production and yields apolymer with ionic end groups, the terminal ionic groups can be renderednon-ionic for use as a gas hydrate formation anddissociation-controlling agent according to the invention.

[0030] The polymerization initiator serves as a polymerizationinitiating radical at the start of polymerization, and promotespolymerization by reaction with the monomer, so that a structure derivedfrom the polymerization initiator is introduced at the polymerizationinitiation end of the polymer. A chain transfer agent-derived structurewill also be included at the polymerization end when a chain transferagent is used for polymerization.

[0031] A polymer used as a gas hydrate formation anddissociation-controlling agent according to the invention has no ionicgroups at its ends, and it may therefore be produced by polymerizationusing non-ionic species for the polymerization initiator and chaintransfer agent. This method is preferred for its low production cost andconvenience. Such polymers may also be produced by using an ionicpolymerization initiator for polymerization and then subsequentlyrendering the ends non-ionic.

[0032] As examples of non-ionic polymerization initiators there may bementioned peroxides, organic peroxy acids, inorganic peroxy acids,non-salt water-soluble azobis compounds having no counter ions,non-water-soluble or poorly water-soluble azobis compounds, or redoxsystems comprising combinations of peroxides and reducing agents.

[0033] As examples of non-ionic polymerization initiators there may bementioned nonionic ones such as 2,2′-azobis(4-methoxy-2,4-dimethylvaleronitrile), 2,2′-azobis(2,4-dimethylvaleronitrile), 2,2′-azobisisobutyronitrile,2,2′-azobis(2-methylbutyronitrile),1,1′-azobis(cyclohexane-1-carbonitrile),2-phenylazo-4-methoxy-2,4-dimethylvaleronitrile,2,2′-azobis[N-(2-carboxyethyl)-2-methylpropionamidine] dihydrate,2,2′-azobis{2-methyl-N-[1,1-bis(hydroxymethyl)-2-hydroxyethyl]propionamide},2,2′-azobis{2-methyl-N-[1,1-bis(hydroxymethyl)ethyl]propionamide},2,2′-azobis[2-methyl-N-(2-hydroxyethyl)propionamide],2,2′-azobis(2-methylpropionamide) dihydrate,2,2′-azobis(2,4,4-trimethylpentane),dimethyl-2,2′-azobis(2-methylpropionate),2,2′-azobis[2-(hydroxymethyl)propionitrile], 4,4′-azobis(4-cyanovalericacid), isobutyl peroxide, α,α′-bis(neodecanoylperoxy)diisopropylbenzene,cumyl peroxyneodecanoate, di-n-propyl peroxydicarbonate, diisopropylperoxydicarbonate, 1,1,3,3-tetramethylbutyl peroxyneodecanoate,bis(4-t-butylcyclohexyl) peroxydicarbonate, 1-cyclohexyl-1-methylethylperoxyneodecanoate, di-2-ethoxyethyl peroxydicarbonate,di(2-ethylhexylperoxy)dicarbonate, t-hexyl peroxyneodecanoate,dimethoxybutyl peroxydicarbonate,di(3-methyl-3-methoxybutylperoxy)dicarbonate, t-butylperoxyneodecanoate, t-hexyl peroxypivalate, t-butyl peroxypivalate,3,5,5-trimethylhexanoyl peroxide, octanoyl peroxide, lauroyl peroxide,stearoyl peroxide, 1,1,3,3-tetramethylbutylperoxy-2-ethyl hexanoate,succinic peroxide, 2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane,1-cyclohexyl-1-methylethylperoxy-2-ethyl hexanoate,t-hexylperoxy-2-ethyl hexanoate, t-butylperoxy-2-ethyl hexanoate,m-toluylbenzoyl peroxide, benzoyl peroxide, t-butylperoxy isobutylate,di-t-butylperoxy-2-methylcyclohexane, 1,1-bis(t-hexylperoxy)-3,5,5-trimethylcyclohexane, 1,1-bis(t-hexylperoxy)cyclohexane,1,1-bis(t-butylperoxy)-3,5,5-trimethylcyclohexane, hydrogen peroxide andthe like. Of these, hydrogen peroxide is preferred.

[0034] As examples of ionic polymerization initiators there may bementioned water-soluble azobis compound initiators. Water-soluble azobiscompound initiators are usually in the form of salts with counter ionsfor water solubility. Polymers obtained using such polymerizationinitiators have structures derived from the polymerization initiators atthe polymerization initiation ends, and thus form salts in water and areionic. However, even such types of polymers can be treated to converttheir terminal ionic groups to non-ionic groups, for use as gas hydrateformation and dissociation-controlling agents according to theinvention.

[0035] Also, while the use of a chain transfer agent is not essential inthe polymerization, a chain transfer agent may also be used. However, asthe structure derived from the chain transfer agent will also beintroduced at the polymer end, a non-ionic species is preferred.Nevertheless, even polymers produced using ionic chain transfer agentscan be later treated to convert their terminal ionic groups to non-ionicgroups, for use as gas hydrate formation and dissociation-controllingagents according to the invention.

[0036] As examples of non-ionic chain transfer agents there may bementioned alkylmercaptans such as n-butylmercaptan and n-octylmercaptan, formaldehyde, acetaldehyde, propionaldehyde, n-butylaldehyde,isobutylaldehyde, diacetyl sulfide, ethylthioglycolate,2-mercaptoethanol, 1,3-mercaptopropanol, 3-mercaptopropane-1,2-diol,1,4-mercaptobutanol, thioglycerin, diethanol sulfide, thiodiglycol,ethylthioethanol, thiourea, allyl alcohol and the like. Preferred amongthese are alkylmercaptans, n-butylaldehyde, isobutylaldehyde, diacetylsulfide, ethylthioglycolate, 2-mercaptoethanol, 1,3-mercaptopropanol and3-mercaptopropane-1,2-diol. Among these preferred agents, hydroxylgroup-containing mercaptans such as 2-mercaptoethanol,1,3-mercaptopropanol and 3-mercaptopropane-1,2-diol are particularlypreferred because the resulting polymers exhibit high gas hydrateformation and dissociation-controlling performance.

[0037] The following is surmised as the reason why polymers withnon-ionic ends are preferred as gas hydrate formation anddissociation-controlling agents. Specifically, it is believed that, asthe polymer ends do not bond with ionic compounds in water or do notvery strongly attract water molecules, a suitable degree of interactionmay be achieved between the gas and polymer in the water, therebyenhancing the gas hydrate formation and dissociation-controllingperformance.

[0038] The reason for the high gas hydrate formation anddissociation-controlling performance of polymers obtained using hydroxylgroup-containing mercaptans is thought to be that the hydroxyl groups atthe polymerization initiation ends and/or polymerization terminationends of such polymers result in an even more suitable interactionbetween the gas or hydrates and the polymer.

[0039] The weight-average molecular weight of the polymer used for theinvention is relatively low, in the range of 500-10,000. A smallerweight-average molecular weight results in greater mobility of thepolymer molecules in water, thereby increasing the gas hydrate formationand dissociation-controlling performance, while a larger molecularweight increases the proportion of formation and dissociationcontrol-exhibiting sites in the polymer.

[0040] While a larger weight-average molecular weight of the polymerwill result in less of a non-ionic effect by the polymer ends, theeffect is significant when the weight-average molecular weight of thepolymer is 10,000 or below. For example, in the case of a polymer with amolecular weight of 10,000 or below obtained by homopolymerization of amonomer with a molecular weight of 100, the polymer will consist of nomore than 100 monomers, such that the proportion of polymerizationinitiation ends will be 1% or greater and will have a major effect.

[0041] As examples of methods for obtaining polymers with weight-averagemolecular weights in the range of 500-10,000 there may be mentioned amethod of using an excess of initiator, a method of using an excess ofchain transfer agent, a method of using both an initiator and a chaintransfer agent, a method of lowering the monomer concentration duringpolymerization, a method of carrying out the polymerization at hightemperature, and a method of carrying out the polymerization at atemperature above the boiling point of the solvent, under pressurizedconditions. Of these, the method of using an excess of initiator, themethod of using an excess of chain transfer agent, and the method ofusing both an initiator and a chain transfer agent are preferred fromthe standpoint of equipment and cost.

[0042] A production method using hydrogen peroxide is especiallypreferred from the standpoint of equipment and cost, as well as thestandpoint of achieving a polymer with high performance as a gas hydrateformation and dissociation-controlling agent. Hydrogen peroxide isthought to function as both a polymerization initiator and a chaintransfer agent. In this case, hydroxyl groups derived from the hydrogenperoxide and solvent-derived end groups from chain transfer to thesolvent are the main groups introduced at the polymer ends, to give apolymer with non-ionic ends.

[0043] As examples of methods of polymerization using hydrogen peroxidethere may be mentioned a method of polymerizing an amphipathic monomeror an amphipathic monomer together with a hydrophilic monomer orhydrophobic monomer which is copolymerizable therewith, by adding theminto the polymerization solvent with hydrogen peroxide. This method ispreferred from the standpoint of obtaining low molecular weight polymersat high yields. The polymerization solvent used for this method ispreferably an alcohol, more preferably a polyfunctional alcohol, andmost preferably ethylene glycol.

[0044] In order to promote dissociation of the hydrogen peroxide, adissociation promoter such as iron, copper, cobalt, manganese, a sulfiteor an amine may be added. Of these, iron, copper, cobalt and manganeseare preferred. It is not preferred to use a compound which producesionic groups at the polymer ends by chain transfer or the like. However,even when a polymer with ionic groups at the polymer ends results, theterminal ionic groups can later be rendered non-ionic for use as a gashydrate formation and dissociation-controlling agent according to theinvention.

[0045] Although the reason that using hydrogen peroxide enhances theperformance as a gas hydrate formation and dissociation-controllingagent is not fully understood, it is conjectured that the presence ofhydroxyl groups at the ends is favorable for control of gas hydrateformation and dissociation.

[0046] The weight-average molecular weight of the polymer used for theinvention may be determined by a publicly known method such asdescribed, for example, in Mori et al., Anal. Chem., 55,2414-2416(1983).

[0047] The gas hydrate formation and dissociation-controlling method ofthe invention comprises adding a gas hydrate formation anddissociation-controlling agent of the invention as described above, to agas hydrate-forming system. Other formation and dissociation-controllingagents may also be added to the gas hydrate formation anddissociation-controlling agent. Examples of formation anddissociation-controlling agents which may be used therewith includehydrophilic polymers, ethylene glycol, triethylene glycol, methanol,ethanol, acetone and the like, but ethylene glycol and hydrophilicpolymers are preferred. When another formation anddissociation-controlling agent is used, the proportion of the gashydrate formation and dissociation-controlling agent of the invention,i.e. the amphipathic polymer having non-ionic ends, including thepolymerization initiation and polymerization termination ends, andhaving a weight-average molecular weight in the range of 500-10,000,will usually be 1-80 wt %, preferably 20-60 wt % and most preferably30-60 wt % in the formation and dissociation-controlling agent.

[0048] Here, a “gas hydrate-forming system” is a system in which a gashydrate-forming substance is dissolved in an aqueous solvent asdescribed, for example, on pages 1-9 of J. Long, A. Lederhos, A. Sum, R.Christiansen, E. D. Sloan; Prep. 73rd Ann. GPA Conv., 1994. In this typeof system, the gas hydrate precipitates as crystals under specificpressure and temperature conditions.

[0049] As gas hydrate-forming substances there may be mentioned gasessuch as carbon dioxide, nitrogen, oxygen, hydrogen sulfide, argon,xenon, methane, ethane or propane, and liquids such as tetrahydrofuran.

[0050] As examples of gas hydrate-forming systems there may be mentioneda system wherein an aqueous phase with a gas such as ethane or propanedissolved in an aqueous solvent such as water or brine is suspended ordispersed in an oily phase such as liquefied gas or crude oil in anatural gas well or oil well, or a system wherein a gas phase such asnatural gas is present in an aqueous phase.

[0051] There are no particular restrictions on the method of adding thegas hydrate formation and dissociation-controlling agent of theinvention to a gas hydrate-forming system, but it is preferably addedafter dissolution in water and/or a water-miscible solvent. Awater-miscible solvent is a solvent that mixes with water in any desiredproportion, and examples thereof include methanol, ethanol, acetone andethylene glycol.

[0052] The lower limit for the amount of addition of the gas hydrateformation and dissociation-controlling agent is preferably at least 0.01part by weight and more preferably at least 1 part by weight withrespect to 100 parts by weight of the free water in the gashydrate-forming system. The upper limit is preferably no greater than100 parts by weight and more preferably no greater than 50 parts byweight. A greater amount of the gas hydrate formation and dissociationcontrolling-agent improves the gas hydrate stabilizing effect, while alesser amount lowers the viscosity of the system, thereby improving thefluidity.

[0053] When a gas hydrate formation and dissociation-controlling agentof the invention is used, various additives such as rust preventives,lubricants, dispersing agents, scaling inhibitors, corrosion inhibitorsand the like may be used in combination therewith.

[0054] The present invention will now be explained in greater detailthrough examples and comparative examples, with the understanding thatthey are in no way limitative on the invention.

[0055] Polymer Molecular Weight Measuring Apparatus

[0056] The polymer molecular weight was measured with the followingapparatus and measuring conditions.

[0057] Apparatus: 8010 System (RI detector) by Toso Corp.

[0058] Column: Shodex GPC KD-806M (8×300 mm) Ultrahydrogel 120 6 μ(8×300 mm)

[0059] Column temperature: 40° C. (thermostat)

[0060] Mobile phase: Dimethylformamide, 0.01 M lithium bromide

[0061] Flow rate: 0.8 ml/min

[0062] Standard polymer for molecular weight

[0063] calculation: Standard polyethylene glycol

[0064] Sample concentration: 0.1 wt % (DMF/LiBr solution)

[0065] Hydrate Formation/Dissociation-Controlling Agent EvaluatingApparatus

[0066] The gas hydrate formation temperature as the index of the gashydrate formation-inhibiting performance of the gas hydrate formationand dissociation-controlling agent, and the gas hydrate dissociationcompletion temperature as the index of the gas hydrate stabilizingperformance, were measured using the apparatus shown in FIG. 1.

[0067] In this apparatus, the high-pressure reaction cell 4 has an innervolume of 100 ml and a normal pressure-resistant design for up to 20MPa. The cell is provided with a gas introduction line 1, a liquidintroduction line 2, a purge line 3, an internal cell thermometer 5, aninternal cell manometer 6 and a reaction cell stirrer 7. The entire cellwas housed inside a thermostat 8 to allow adjustment of the internalcell temperature by the temperature of the thermostat 8. Thehigh-pressure reaction cell 4 is provided with 3 cm-diameter observationports (not shown) at three locations to allow the condition in the cellto be observed.

[0068] Formation-Inhibiting Performance Evaluation Method

[0069] The gas hydrate formation-inhibiting performance was evaluated inthe following manner. Specifically, a 0.5 wt % aqueous solution of thegas hydrate formation and dissociation-controlling agent to be evaluatedwas introduced through the liquid introduction line 2 of the apparatusshown in FIG. 1, methane gas was introduced through the gas introductionline 1 to an internal cell pressure of 10 MPa, and the internal celltemperature was set to 20° C., a definite higher temperature than theformation equilibrium temperature of the methane hydrate at thatpressure. The internal cell temperature was then slowly lowered at −4°C./hr while stirring the cell contents, and the state of methane hydrateformation in the cell at a given temperature was observed. The internalcell pressure was lowered by the methane hydrate formation, while thegas hydrate production slightly increased the internal cell temperaturesince it is an exothermic reaction. A lower internal cell temperaturewhen the pressure begins to significantly fall, i.e. a lower methanehydrate formation temperature, was interpreted as greater gas hydrateformation-inhibiting performance.

[0070] Equilibrium Stabilization Performance Evaluation Method

[0071] The equilibrium stabilization performance was evaluated in thefollowing manner. Specifically, after measuring the methane hydrateformation temperature in the performance evaluation described above, thethermostat temperature was lowered to 2° C. below the methane hydrateformation initiation temperature, and the gas was allowed to stand untilthe internal cell pressure and internal cell temperature becameconstant. When the internal cell temperature was then increased at 4°C./hr, the methane hydrate began to gradually dissociate inside thecell, finally separating completely into water and methane gas. A higherinternal cell temperature, i.e. a higher methane hydrate dissociationcompletion temperature, was interpreted as greater gas hydrateequilibrium stabilization performance.

[0072] Kinetic Stabilization Performance Evaluation Method

[0073] The kinetic stabilization performance whereby the gas hydratedissociation rate is kinetically reduced to delay gas hydratedissociation was evaluated in the following manner. Specifically, afterproducing methane hydrate by the same procedure as for measurement ofthe methane hydrate formation temperature by the formation-inhibitingperformance evaluation described above, the thermostat temperature wasset to 2° C. and the gas was allowed to stand until the internal cellpressure and internal cell temperature became constant. Next, themethane gas in the cell was evacuated to an internal cell pressure of 2MPa. The cell was sealed in this state, and the time until a constantinternal cell pressure was reached was measured. A longer time(hereinafter referred to as “kinetic dissociation delay time”) wasinterpreted as greater kinetic stabilization performance for methanehydrate dissociation.

EXAMPLE 1

[0074] After adding 120 g of 1,4-dioxane and 60 g ofN-isopropylmethacrylamide (Mitsubishi Rayon) as an amphipathic monomerto a 300 ml separable flask equipped with a stirrer, condenser tube,nitrogen-introduction tube and thermocouple, and dissolving the monomer,nitrogen bubbling was performed for 30 minutes at a flow rate of 200ml/min to remove the dissolved oxygen. The temperature in the flask wasthen raised to 80° C., after which there was added a solution of 3 g of2,2′-azobisisobutyronitrile (V-60, Wako Pure Chemical Industries) as apolymerization initiator dissolved in 20 g of 1,4-dioxane, andpolymerization was initiated. Polymerization was conducted whilestirring under a nitrogen stream at a flow rate of 100 ml/min, and thereaction was continued for 6 hours at 80° C. This was allowed to cool,and 150 g of tetrahydrofuran was added to dilute the polymerizationsolution, which was then added dropwise to 3 L of n-hexane whilestirring. After filtering off the obtained polymer, a vacuum drier wasused at 60° C. for drying under reduced pressure overnight, to obtain 40g of N-isopropylmethacrylamide polymer as a white powder. Theweight-average molecular weight of the polymer was 4800 in terms ofstandard polyethylene glycol.

[0075] A gas hydrate formation and dissociation-controlling agentcomprising the N-isopropylmethacrylamide polymer obtained in this mannerwas diluted with distilled water to a solid concentration of 0.5 wt %and, upon measuring the gas hydrate formation anddissociation-controlling performance, the gas hydrate formationtemperature was 4° C., the gas hydrate dissociation completiontemperature was 22° C., and the kinetic dissociation delay time was 530minutes.

EXAMPLES 2-5

[0076] Polymerization and gas hydrate formation anddissociation-controlling performance evaluations were conducted by thesame procedure as in Example 1. The monomers, initiators, chain transferagents, weight-average molecular weights, gas hydrate formationtemperatures, gas hydrate dissociation completion temperatures andkinetic dissociation delay times are shown in Table 1. TABLE 1 HydrateWeight- Hydrate dissociation Kinetic Monomer type Chain averageformation completion dissociation (Compositional) transfer moleculartemperature temperature delay time ratio) Initiator agent weight (° C.)(° C.) (min) Example IPMA(100) V-60 — 4,800 4.0 22.0 530 1 ExampleIPMA(100) V-59 nBM 2,400 4.0 23.0 540 2 Example IPMA(100) V-59 MEt 2,3003.5 24.9 600 3 Example DEAA(100) V-60 MEt 4,000 4.0 20.0 500 4 ExampleVCap(100) V-60 MEt 2,800 5.0 18.0 420 5

EXAMPLE 6

[0077] After adding 250 g of ethylene glycol to a 1000 ml volatilesubstance-removable separable flask equipped with a stirrer,nitrogen-introduction tube and thermocouple, nitrogen bubbling wasperformed for 30 minutes at a flow rate of 200 ml/min, and the mixturewas heated to 120° C. A mixture of 120 g of N-isopropylmethacrylamide(Mitsubishi Rayon) as an amphipathic monomer, 200 g of methanol and108.2 g of 30 wt % hydrogen peroxide was added dropwise thereto over aperiod of 3 hours, for polymerization of the mixture. After completionof the dropwise addition, polymerization was continued for 1 hour at120° C., to obtain 410 g of polymer solution. The polymerization wasconducted while stirring under a nitrogen stream and removing thevolatile substance composed mainly of methanol. This was allowed tocool, and 300 g of tetrahydrofuran was added to dilute thepolymerization solution, which was then added dropwise to 6 L ofn-hexane while stirring. After filtering off the obtained polymer, avacuum drier was used at 60° C. for drying under reduced pressureovernight, to obtain 90 g of N-isopropylmethacrylamide polymer as awhite powder.

[0078] As a result of measuring the molecular weight using a GPC with adimethylformamide/lithium bromide solution as the mobile phase, theweight-average molecular weight of the polymer was found to be 700 interms of standard polyethylene glycol.

[0079] The gas hydrate formation and dissociation-controllingperformance of the polymer obtained in this manner was evaluated by thesame procedure as in Example 1. The monomer, initiator, initiatoramount, weight-average molecular weight, gas hydrate formationtemperature, gas hydrate dissociation completion temperature and kineticdissociation delay time are shown in Table 2.

EXAMPLE 7

[0080] An N-isopropylmethacrylamide polymer was obtained bypolymerization in the same manner as Example 6, except that the amountof hydrogen peroxide used was changed to 27.1 g. The gas hydrateformation and dissociation-controlling performance of the obtainedpolymer was evaluated by the same procedure as in Example 1. Themonomer, initiator, initiator amount, weight-average molecular weight,gas hydrate formation temperature, gas hydrate dissociation completiontemperature and kinetic dissociation delay time are shown in Table 2.

EXAMPLE 8

[0081] An N-isopropylmethacrylamide polymer was obtained bypolymerization in the same manner as Example 6, except that the dropwiseaddition and polymerization of the mixture of N-isopropylmethacrylamide,methanol and hydrogen peroxide for 3 hours was changed to 1 hour, andthe continuation of polymerization for 1 hour after the dropwiseaddition was changed to 3 hours. The gas hydrate formation anddissociation-controlling performance of the obtained polymer wasevaluated by the same procedure as in Example 1. The monomer, initiator,initiator amount, weight-average molecular weight, gas hydrate formationtemperature, gas hydrate dissociation completion temperature and kineticdissociation delay time are shown in Table 2.

EXAMPLE 9

[0082] An N-isopropylmethacrylamide polymer was obtained bypolymerization in the same manner as Example 6, except that FeSO₄.7H₂Owas added to the ethylene glycol at 200 ppm in terms of Fe, the reactiontemperature was 80° C., and an N-isopropylmethacrylamide powder andhydrogen peroxide were simultaneously added over a period of 3 hours,with no methanol. The gas hydrate formation and dissociation-controllingperformance of the obtained polymer was evaluated by the same procedureas in Example 1. The monomer, initiator, initiator amount, dissociationpromoter, weight-average molecular weight, gas hydrate formationtemperature, gas hydrate dissociation completion temperature and kineticdissociation delay time are shown in Table 2. TABLE 2 Hydrate MonomerInitiator Weight- Hydrate dissociation Kinetic type amount averageformation completion dissociation (Compositional (initiator/Dissociation molecular temperature temperature delay time ratio)Initiator monomer) promoter weight (° C.) (° C.) (min) Example IPMA(100)H₂O₂ 0.27 — 700 3.6 25.1 600 6 Example IPMA(100) H₂O₂ 0.068 — 1,500 3.725.5 620 7 Example IPMA(100) H₂O₂ 0.034 — 3,400 3.5 25.9 630 8 ExampleIPMA(100) H₂O₂ 0.10 FeSO₄ 2,200 3.6 25.8 600 9

EXAMPLE 10

[0083] After adding 120 g of 1,4-dioxane, 57 g ofN-isopropylmethacrylamide (Mitsubishi Rayon) as an amphipathic monomerand 3 g of acrylamide to a 300 ml separable flask equipped with astirrer, condenser tube, nitrogen-introduction tube and thermocouple,and dissolving the monomer, nitrogen bubbling was performed for 30minutes at a flow rate of 200 ml/min to remove the dissolved oxygen. Thetemperature in the flask was then raised to 80° C., after which therewas added a solution of 3 g of 2,2′-azobisisobutyronitrile (v-60, WakoPure Chemical Industries) as a polymerization initiator dissolved in 20g of 1,4-dioxane, and polymerization was initiated. Polymerization wasconducted while stirring under a nitrogen stream at a flow rate of 100ml/min, and the reaction was continued for 5 hours at 80° C. This wasallowed to cool, and 150 g of tetrahydrofuran was added to dilute thepolymerization solution, which was then added dropwise to 3 L ofn-hexane while stirring. After filtering off the obtained polymer, avacuum drier was used at 60° C. for drying under reduced pressureovernight, to obtain 28 g of a white powder.

[0084] As a result of measuring the molecular weight using a GPC with adimethylformamide/lithium bromide solution as the mobile phase, theweight-average molecular weight of the polymer was found to be 3100 interms of standard polyethylene glycol.

[0085] A gas hydrate formation and dissociation-controlling agentcomprising the N-isopropylmethacrylamide/acrylamide copolymer obtainedin this manner was diluted with distilled water to a solid concentrationof 0.5 wt %, and upon measuring the gas hydrate formation anddissociation-controlling performance, the gas hydrate formationtemperature was 5° C., the gas hydrate dissociation completiontemperature was 17.9° C., and the kinetic dissociation delay time was490 minutes.

EXAMPLES 11-13

[0086] Polymerization and gas hydrate formation anddissociation-controlling performance evaluation were conducted by thesame procedure as in Example 10. The monomers, initiators, chaintransfer agents, weight-average molecular weights, gas hydrate formationtemperatures, gas hydrate dissociation completion temperatures andkinetic dissociation delay times are shown in Table 3. TABLE 3 HydrateWeight- Hydrate dissociation Kinetic Monomer type Chain averageformation completion dissociation (Compositional transfer moleculartemperature temperature delay time ratio) Initiator agent weight (° C.)(° C.) (min) Example IPMA/AAm(90/10) V-60 — 3,100 5.0 17.9 490 10Example IPMA/THFMA(90/10) V-59 nOM 7,800 5.0 18.5 405 11 ExampleDEAA/AAm(70/30) V-59 — 1,800 5.5 19.0 420 12 Example VCap/HEMA(80/20)V-60 nOM 5,600 5.1 17.5 400 13

EXAMPLE 14

[0087] Polymerization and gas hydrate formation anddissociation-controlling performance evaluation were conducted by thesame procedure as in Example 8, except that the amphipathic monomerswere changed to 114 g of N-isopropylmethacrylamide (Mitsubishi Rayon)and 6 g of acrylamide. The monomer, initiator, initiator amount,weight-average molecular weight, gas hydrate formation temperature, gashydrate dissociation completion temperature and kinetic dissociationdelay time are shown in Table 4. TABLE 4 Hydrate Monomer InitiatorWeight- Hydrate dissociation Kinetic type amount average formationcompletion dissociation (Compositional (initiator/ molecular temperaturetemperature delay time ratio) Initiator monomer) weight (° C.) (° C.)(min) Example IPMA/AAm(90/10) H₂O₂ 0.034 3,800 4.5 18.6 520 14

EXAMPLE 15

[0088] After adding 80 g of N-isopropylmethacrylamide (MitsubishiRayon), as an amphipathic monomer, to a 1000 ml separable flask equippedwith a stirrer, condenser, nitrogen-introduction tube and thermocouple,and heating to 90° C., there were added 0.24 g of 2,2′-azobis(2-methylbutyronitrile) (V-59, product of Wako Pure ChemicalIndustries), 0.12 g of 2,2′-azobis(cyclohexane-1-carbonitrile (V-40,product of Wako Pure Chemical Industries) and 6 g of 2-mercaptoethanolwhile stirring, and then polymerization was conducted for 4 hours whilestirring at 90-100° C. The product was allowed to cool, and 200 g oftetrahydrofuran was added to dilute the polymerization solution, whichwas then added dropwise to 6 L of n-hexane while stirring. Afterfiltering off the obtained polymer, a vacuum drier was used at 60° C.for drying under reduced pressure overnight, to obtain 60 g ofN-isopropylmethacrylamide polymer as a white powder.

[0089] As a result of measuring the molecular weight using a GPC with adimethylformamide/lithium bromide solution as the mobile phase, theweight-average molecular weight of the polymer was found to be 2300 interms of standard polyethylene glycol.

[0090] The gas hydrate formation and dissociation-controllingperformance of the polymer obtained in this manner was evaluated by thesame procedure as in Example 1. The monomer, initiator, chain transferagent, weight-average molecular weight, gas hydrate formationtemperature, gas hydrate dissociation completion temperature and kineticdissociation delay time are shown in Table 5. TABLE 5 Hydrate Weight-Hydrate dissociation Kinetic Monomer type Chain average formationcompletion dissociation (Compositional transfer molecular temperaturetemperature delay time ratio) Initiator agent weight (° C.) (° C.) (min)Example IPMA(100) V-59/V-40 MEt 2,500 3.8 25.0 610 15

COMPARATIVE EXAMPLE 1

[0091] After adding 120 g of distilled water and 60 g ofN-isopropylmethacrylamide (Mitsubishi Rayon), as an amphipathic monomer,to a 300 ml separable flask equipped with a stirrer, condenser tube,nitrogen-introduction tube and thermocouple, nitrogen bubbling wasperformed for 30 minutes at a flow rate of 200 ml/min to remove thedissolved oxygen. The temperature in the flask was then raised to 80°C., after which there was added a solution of 5 g of2,2′-azobis(2-methylpropionamide) dihydrochloride (V-50, Wako PureChemical Industries) as a polymerization initiator dissolved in 20 g ofdistilled water, and polymerization was initiated.N-isopropylmethacrylamide only dissolves in water to approximately 10%at 80° C. but, as the polymer becomes more water-soluble aspolymerization proceeds, the amount of undissolved monomer tends todecrease. On the other hand, poly (N-isopropylmethacrylamide) has alower critical solution temperature of approximately 42° C., and it isinsoluble in water at 80° C. The solution therefore becomes cloudyduring polymerization. Polymerization was conducted while stirring undera nitrogen stream at a flow rate of 100 ml/min, and the reaction wascontinued for 5 hours at 80° C. This was allowed to cool, and 150 g ofacetone was added to dilute the polymerization solution, which was thenconcentrated to dryness with a rotary evaporator. The obtained crudepolymer was redissolved in 300 ml of acetone, and the solution was addeddropwise to 3 L of n-hexane while stirring. After filtering off theobtained polymer, a vacuum drier was used at 60° C. for drying underreduced pressure overnight, to obtain 32 g of N-isopropylmethacrylamide/acrylamide copolymer as a white powder. The weight-average molecularweight of the copolymer was 4000 in terms of standard polyethyleneglycol.

[0092] A gas hydrate formation and dissociation-controlling agentcomprising the N-isopropylmethacrylamide polymer obtained in this mannerwas diluted with distilled water to a solid concentration of 0.5 wt %,and upon measuring the gas hydrate formation anddissociation-controlling performance, the gas hydrate formationtemperature was 6° C., the gas hydrate dissociation completiontemperature was 16.5° C., and the kinetic dissociation delay time was220 minutes.

COMPARATIVE EXAMPLES 2-4

[0093] Polymerization and gas hydrate formation anddissociation-controlling performance evaluation were conducted by thesame procedure as in Example 1 for Comparative Example 2 and by the sameprocedure as in Comparative Example 1 for Comparative Examples 3 and 4.The monomers, initiators, chain transfer agents, weight-averagemolecular weights, gas hydrate formation temperatures, gas hydratedissociation completion temperatures and kinetic dissociation delaytimes are shown in Table 6. TABLE 6 Hydrate Weight- Hydrate dissociationKinetic Monomer type Chain average formation completion dissociation(Compositional transfer molecular temperature temperature delay timeratio) Initiator agent weight (° C.) (° C.) (min) Comp. Ex. 1 IPMA(100)V-50 — 4,000 6.5 15.5 220 Comp. Ex. 2 IPMA(100) V-60 — 38,000 5.5 17.5350 Comp. Ex. 3 DEAA/AAm(70/30) VA-044 Met 5,800 5.5 17.0 300 Comp. Ex.4 Vcap/HEMA(80/20) VA-044 — 76,000 6.1 17.2 330

[0094] Industrial Applicability

[0095] The gas hydrate formation and dissociation-controlling agents ofthe invention have an inhibiting effect to control formation of gashydrates under conditions in which gas hydrates form and an effect ofequilibrium and kinetic stabilization of gas hydrates under conditionsin which gas hydrates undergo gradual dissociation. By using gas hydrateformation and dissociation-controlling agents of the invention havingboth of these effects, it is possible to achieve effective control overgas hydrate formation and dissociation.

1. A gas hydrate formation and dissociation-controlling agent which isan amphipathic polymer having non-ionic polymerization initiation andpolymerization termination ends of the polymer, and having aweight-average molecular weight in the range of 500-10,000.
 2. Acontrolling agent according to claim 1, wherein the amphipathic polymeris obtained by (co) polymerization of an amphipathic monomer.
 3. Acontrolling agent according to claim 1 or 2, wherein either or both thepolymerization initiation and polymerization termination ends of saidpolymer are hydroxyl groups.
 4. A controlling agent according to claim3, wherein said polymer is polymerized or copolymerized using hydrogenperoxide as the polymerization initiator.
 5. A controlling agentaccording to any one of claims 1 to 4, wherein said polymer ispolymerized or copolymerized in the presence of a hydroxylgroup-containing mercaptan.
 6. A controlling agent according to any oneof claims 2 to 5, wherein said polymer is obtained by (co)polymerizationof N-isopropylmethacrylamide.
 7. A gas hydrate formation anddissociation-controlling method which comprises adding a gas hydrateformation and dissociation-controlling agent according to any one ofclaims 1 to 6 to a gas hydrate-forming system.
 8. A method according toclaim 7, wherein the gas hydrate formation and dissociation-controllingagent is added to the gas hydrate-forming system as a solution dissolvedin water and/or a water-miscible solvent.
 9. A method according to claim7 or 8, wherein the amount of the gas hydrate formation anddissociation-controlling agent added is 0.01 to 100 parts by weight withrespect to 100 parts by weight of free water in the gas hydrate-formingsystem.